Hello Marko:
In the case of your device (MK10FN1M0VLQ12), SPI pins will be in high impedance state, so I think you can safely apply 3.3 V. :smileyalert: IMPORTANT -> This is NOT true for pure analog pins, since those have ESD clamping diodes to VDD, and if you apply voltage there will be a sink current and most probably the MCU will run.
Having said that, although it should be safe to drive voltage on digital pins when MCU is unpowered, it is not recommended, since you do not have complete knowledge of the general internal state of the MCU, such as the analog pins behavior mentioned above. A better solution would be to power the MCU and put it in a low power mode such as VLLS1 when you do not need it. This way you have control of the ports state (e.g. Disabled/High-Z) while maintaining low current consumption.
I hope this helps.
Regards!,
Jorge Gonzalez
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